The present invention relates to a ground penetrating radar system and method for detecting an object on or buried in the ground. More specifically, the present invention relates to a ground penetrating radar system for detecting objects on or below a ground surface, comprising at least one transmit antenna having a first foot print at the ground surface at least one receive antenna having a second foot print at the ground surface, and processing means connected to the at least one transmit antenna and the at least one receive antenna. Such a system may primarily be used to detect buried mines, both metallic and non-metallic.
American patent U.S. Pat. No. 5,680,048 describes a mine detecting device using a combination of sensor systems to detect mines dispersed on or buried in the ground. In the system, a combination is used of a ground penetrating radar, a radiometer and a metal detector. The ground penetrating radar comprises processing means and a combined transmit/receive antenna. The transmit and receive antenna may also be separate antennas. In the system disclosed, the transmit antenna is combined with a dielectric lens to generate a collimated beam to obtain a beam with nearly constant power for short distances.
In general, a ground penetrating radar system directs an electromagnetic signal (such as a pulse) by means of the transmit antenna into the ground, and the back scattered signal is received by the receive antenna and useful information regarding objects is extracted from the received signal. When the signal transmitted is of a amplitude, frequency and duration, it is possible to separate useful information from noise. However, when a signal is transmitted with a trailing resonance signal (such as generated by certain shapes of transmit antennas) it is not clear what part of tho back scatter signal contains the useful information.
Ground penetrating radar systems usually operate in the microwave frequency range, and numerous designs of transmit and receive antennas have been developed to overcome specific problems, such as transmission and reception in certain environments, transmitting specific polarisation signals and broad band signals having specific phase and polarisation characteristics.
A problem with the described ground penetrating radar system and other known ground penetrating radar systems is that the resolution and depth of penetration is usually not sufficient to provide adequate operation. Especially in a dielectrically cluttered region environment, the effective application of the ground penetrating radar system might be impaired.
It is therefore an object of the present invention to provide a ground penetrating radar system with a good performance in detecting objects on or buried in the ground.
This object is achieved by a ground penetrating radar system of the type defined in the preamble, in which the first and second foot print have different dimensions. With foot print at the ground interface, the plan coverage of the antenna on the ground surface is meant. This arrangement will result in reception of a signal, which is significantly reduced in noise, allowing easier processing of the received signal to detect the object on or in the ground.
Preferably, the second foot print is enclosed by the first foot print. This allows illumination by the transmit antenna of a larger area at the ground surface and a very localised sensing of back scattered signals by the receive antenna.
In a further embodiment of the ground penetrating radar system, the at least one transmit antenna is arranged to operate as a far field antenna and the at least one receive antenna is arranged to operate as a near field antenna. Objects on or buried in the ground are illuminated by the transmit antenna, and the back scattered signals are sensed very locally by the receive antenna. Hereby, only back scatter signals will be received from objects in the direct vicinity of the receive antenna, making the back scatter signal less noisy and less polluted with clutter from other objects or material.
In a further embodiment of the ground penetrating radar system the at least one transmitter antenna and the processing mews are arranged to provide a pulse shape with reduced ringing characteristics,
The transmit antenna of the present ground penetrating radar system is preferably a TEM horn antenna. In a preferred embodiment, the transmit antenna produces a single-cycle pulse of RF energy directed towards the ground surface. Such a transmit antenna allows generation of a single cycle pulse with a very pure foil, i.e. without a trailing resonance signal. This trailing resonance signal is usually referred to as ring. Transmitting a pure single-cycle pulse without ringing will result in a back scatter signal with much more useful information regarding the object to be detected, as no back scatter signal will be present form the trailing resonance signal. A pure single-cycle pulse will result in a cleaner turn signal, allowing the use of early time event(s) within the radiated waveform e.g. the leading edge of the pulse, as very accurate markers in both the transmitted and received signal. This enables a straightforward subtraction of signals in the time domain and signal calibration functions.
However, it is also possible to obtain a maximum backscatter signal from the buried object with other transmitted pulse shapes. For this, either he processing means, the transmit antenna or both should be adapted. It is e.g. possible to change the transmit horn antenna geometric profile and/or the characteristics of a pulse generator comprised in the processing means
It is even possible to use frequency modulated or stopped frequency continuous waveforms for the transmitted signal to be radiated by the at least one transmit antenna. However, as these techniques are acausal synthetic pulse techniques, different radiation mechanisms may be used, and consequently other specific types of transmit antenna Examples of such different types of transmit antenna may be other horn antennae, planar or conic spiral antannae.
In a preferred embodiment, the at least one transmit antenna comprises a first and a second side wall, attached to a connector at a first end, the first and second side wall being at a predetermined and to each other, the first and second side wall each comprising a plural of sections with a predetermined length and a predetermined width, the predetermined width of at least one of the plurality of sections tapering from the side nearest to the first end of the first and second side wall towards the side nearest to a second end of the first and second side wall. This arrangement of the transmit antenna allows generation of a very pure single-cycle pulse.
In a further embodiment, the at least one receive antenna has a low sensitivity. By only sensing the strongest signals, signals are received, which comprise information concerning objects of interest only. Noise in the received signal is reduced, as well as clutter form other objects which are tot of interest. The sensitivity of a receive antenna is the product of the frequency dependence of its effective length and impedance match to the receiver. The sensitivity of the receive antenna is selected such that only local back scattered signal is recovered, as scatter signal from more distant object also illuminated by the transmit antenna is very weak.
Preferably, the at least one receive antenna is a waveguide aperture antenna. Alternatively, the at least one receive antenna is a small loop or short dipole antenna preferably a xcex/10-antenna.
In a further embodiment of the ground penetrating radar system according to the present invention, the at least one receive antenna comprises an additional receive antenna with an additional receive antenna foot print smaller than the transmit antenna foot print. This more directively focused additional antenna has a sensitivity to provide a deeper penetration of the ground, allowing detection of buried objects, which are buried more deeply in the ground. By selecting the proper receiving characteristic of the receive antenna and the additional receive antenna, the region of interest in the ground can be property chosen.
In a still further embodiment, the at least one receive antenna comprises an array of receive antenna. This arrangement allows a mole complex array processing of signals received from each of the receive antennae of the array. Integrating the signals from the multiple near-field receive antennae enables an enhanced focusing, resulting in a better resolution of the image formed from the sub-surface ground and hence a better detectability of buried objects.
The at least one transmit antenna and the at least one receive antenna may be attached to a moving platform. This arrangement allows a continuous scan along the ground while also recording the exact location. Using synthetic aperture radar techniques provides an additional increase in azimuthal and range resolution along the plane of the array directed into the ground.
A second aspect of the present invention relates to a method for detecting an object on or below a ground surface, comprising the steps of illuminating the ground surface with a wide band RF signal, the wide band RF signal being transmitted by a transmit antenna having a first foot print, in which wideband RF signals reflected by the object are probed by at least one receive antenna having a second foot print, the first and second foot print having different dimensions. This method will result in reception of reflected signals, which are significantly reduced in noise, allowing easier processing of the received signal to detect the object on or in the ground.
Preferably, the transmit antenna is being operated in substantially a near field mode and the at least one receive antenna is operated in substantially a near field mode. Objects on or buried in the ground are then illuminated by the transmit antenna, and the back scattered signals are sensed very locally by the receive antenna. Hereby, only back scatter signals will be received from objects in the direct vicinity of the receive antenna, making the back scatter signal less noisy and less polluted with clutter from other objects or material.
In a further embodiment of the present method, the reflected wideband RF signals are probed by a linear array of receive antennae, enabling further signal processing of the received signals to enhance the resolution and detectability.
The wideband RF signal is transmitted substantially as a pulse shape with reduced ringing characteristics, preferably as a single-cycle of RF energy, resulting in a back scatter signal with much more useful information regarding the object to be detected, as no back scatter signal will be present form a trailing resonance signal.